1. Field of the Invention
The present invention relates to a catheter assembly for monitoring intracranial fluid pressure and removing intracranial fluid, and a method of use.
2. Description of the Prior Art
The skull is a bony housing of fixed volume containing three types of matter--blood, brain and cerebrospinal fluid--each of which occupies a portion of that volume. If the portion of the volume of one of the three increases without a concomitant fall in the portions of the other two, the intracranial pressure increases. Because the brain has a limited capacity to adapt to increases in intracranial volume, once the limit has been met, small increases in volume cause significant increases in intracranial pressure.
Maintaining cerebral blood flow to provide adequate oxygen and glucose to the brain is critical. However, an increase in the volume of the brain or of cerebral spinal fluid as the result of trauma, and the like, correspondingly constricts the flow of blood and can stop it completely if the increase in fluid is large enough to herniate the brain at the base of the skull. In such cases, death results.
Because it is not practical to monitor cerebral blood flow at the bedside, an algorithm based upon the relationship between mean arterial pressure, intracranial pressure and the cerebral perfusion pressure is used to calculate blood flow in the brain (CPP=MAP-ICP). Thus, a satisfactory means for monitoring intracranial pressure by placement of a pressure transducer into the intraventricular, subarachnoid or intraparentchymal spaces is of utmost importance to the management of head injury cases and has long been sought.
Assemblies for monitoring pressure at various locations within the body are known. The earliest pressure monitoring devices utilize a pressure sensitive diaphragm in contact with a column of sterile fluid contained within a catheter inserted into the blood vessel, the brain, or other area containing a fluid pressure of interest. Pressure exerted by the fluid, for instance the cerebrospinal or cephalorachitic liquid, is transmitted through the fluid column within the catheter to an external pressure transducer that transforms the pressure signal into an analog or digital form suitable for readout on a monitoring device, such as that commonly used to monitor blood pressure. However, pressure monitoring devices that utilize a column of fluid are easily contaminated with bacteria or air bubbles. Air bubbles in the line distort the pressure readings and bacterial contamination of the fluid may inadvertently expose the patient to sepsis.
The fluid coupled systems used to monitor intracranial pressure access the compartments of the brain by means of a ventricular catheter or bolts placed through the skull. Since the pressures to be monitored are relatively low (0-50 mm Hg), the hydrostatic effects of the fluid column can compromise the readings. Additionally, the fluid column can affect the frequency response of the system.
To eliminate the risks and disadvantages inherent in catheters employing a fluid column for transmitting the pressure reading, improved pressure monitoring devices have been developed that couple the pressure sensitive diaphragm located at the distal end of the catheter with an electrical or optical means for generating a pressure signal and transmitting it to the proximal end of the catheter, and thence to the pressure transducer and monitor. The electrical pressure monitoring diaphragms are typically fitted with a miniaturized Wheatstone bridge strain gauge comprising a series of resistors whose resistance is modified in proportion to the distance from the zero position the diaphragm of the pressure sensor is displaced by the applied pressure. Electrical pressure sensors are commonly employed in hospitals for continuous monitoring of blood pressure and the like. For this reason, hospitals employ monitors adapted to receiving an electrical output from the Wheatstone bridge pressure sensor and transforming it into a pressure reading using well known technology. Thus, an intracranial pressure monitoring catheter employing an electrical pressure sensor could be plugged directly into the pressure, monitor found in most hospital rooms without the need for an expensive intervening transducer to modify the signal into a format compatible with the monitor.
However, in designing a pressure monitoring assembly for monitoring intracranial pressure, special considerations are required. Inherent in all electrical pressure monitoring sensors is a risk of electrical shock that may render them unsuitable for insertion into the interior of the brain. Introducing electrical currents into the brain risks permanent damage.
Optical pressure monitoring transducers avoid this risk. Optical pressure sensors generally employ a light reflective diaphragm placed at the distal tip of an optical fiber. Displacement of the reflective diaphragm by applied pressure changes the intensity and/or other spectral characteristics of the reflected light signal, depending upon the type of reflective sensor used. For instance, U.S. Pat. No. 5,065,010 to Knute, issued Nov. 12, 1991, discloses an optical pressure catheter having a set of optical fibers for transmitting a light beam to and from a transducer which modulates the intensity of the reflected light in accordance with the sensed pressure. A photosensor comprising a bellows compressible by pressure is located at the distal end of the catheter and a photodetector located at the proximal end of the catheter measures the modulated intensity of the returned beam and produces a corresponding measurement signal. However, optical transducers operating upon the principles of intensity modulation suffer from the drawback that any curvature of the fiber optic extraneously reduces the intensity of the reflected light.
To overcome this source of error the Camino catheter preferably also contains a second set of optical fibers for transmitting a reference light beam to and from the location of the sensor. The reference light beam is sent to a second photosensor that measures the intensity of the returned reference light beam and produces a correction signal that compensates for variations in transmittance caused by bending of the catheter.
One of the disadvantages of the Camino system for monitoring intracranial pressure is that a dedicated stand alone interface module, such as that manufactured by Camino Laboratories, is required to display the pressure and communicate with various commercial patient monitors. Zeroing is also dependent upon the interface module, which "reads" the characteristics of the individual sensor and provides for zeroing by means of a screw type adjustment. Additionally, to reduce the error caused by bending the optical fibers, intracranial pressure catheters that rely upon modulated intensity of the reflected beam must be very rigid in construction and are therefore inserted into the skull via a bolt. The most reliable intensity modulation catheters, since they require four optical fibers, are larger, more invasive, and therefore inherently more dangerous, than is desirable.
Optical pressure transducers that modulate the wavelength of the reflected light in accordance with the variable to be measured are also known. For instance, U.S. Pat. Nos. 4,329,058 and 4,678,904, which are hereby incorporated by reference in their entirety, describe an optical transducer having an optically resonant sensor by which the wavelength of the reflected light is modified if the reflective diaphragm is deflected by applied pressure from its zero position. This kind of pressure transducer incorporates a Fabry-Perot interferometer in the reflective sensor.
The Fabry-Perot interferometer operates according to well-known principles whereby the gap between two reflective surfaces causes a plurality of reflections and splittings of a single beam of incident light, such that constructive and destructive interference of the components of the incident light beam may occur numerous times. Inasmuch as an inherent phase reversal occurs when light is reflected from a more dense medium to a less dense medium (for example, when it passes through the diaphragm of the optical sensor to the air or other medium in the Fabry-Perot gap), it is possible for the main reflected light beams to cancel in a gap having a width equal to a multiple of half wavelengths of the incident light. Light beams transmitted through the Fabry-Perot gap and the surface of the sensor (those not having half wavelengths in multiples of the gap width) undergo an even number of reflections, so that, in the event of the phase reversal above described, the even number of phase reversals produces no net phase reversal. These light beams transmitted through the gap are in constructive interference with each other and are therefore transmitted through the gap and returned to the photosensor assembly for processing. Under conditions of high reflectivity, even a small variation in the frequency of light caused by passage through the Fabry-Perot interferometer dramatically reduces transmission of the frequency-altered light beams.
Based upon these principles (See Handbook of Physics, 2d, published by McGraw-Hill Section 7, Chapter 5, Part 6 for further information), a Fabry-Perot sensor having a displaceable diaphragm can be used to monitor a physical variable such as applied pressure, temperature, gas density or pH value. In operation, the gap width varies as a function of the physical parameter to be measured. Therefore, the width in the gap corresponding to an applied pressure can be used to measure the pressure. As is described in full detail in U.S. Pat. No. 4,908,474, photodetector circuits can readily compare the incident and reflected light beams to determine the width of the Fabry-Perot gap and compute the magnitude of the sensed variable therefrom. For example, the Model 1400 multisensor system manufactured by Metricor (Woodenville, Wash.) can be used with a Fabry-Perot sensor to detect pressure readings generated by as little as a single Angstrom of change in the gap width.
The changes in intensity or in wavelength in reflected light beams can be converted to a signal compatible with conventional monitors for display as a digital readout or printout by a suitable pressure transducer. However, known pressure transducers having this capability, such as those useful for monitoring intracranial pressure are expensive and cumbersome. For instance, U.S. Pat. Nos. 4,611,600, 4,703,174, and 4,705,047 disclose various types of transducer circuits suitable for receiving reflected light beams and processing them to yield signals to the monitor that indicate the value of the parameter of interest. However, there is great need for an inexpensive optical pressure transducer, preferably one employing modern techniques of microcircuitry, that operates off the output voltage of the common hospital pressure monitor and produces a modified electrical signal (such as that supplied by the output from a Wheatstone bridge electrical strain gauge) suitable for input to the same bedside monitor.
Intracranial pressure monitoring devices are needed to monitor dangerously high intracranial pressures. One means of reducing elevated intracranial pressure is to drain off cerebrospinal fluid. Thus, a need exists, not only for intracranial pressure monitors, but also for devices for draining fluid from the brain, without inflicting unnecessary damage.
The pressure monitoring device must be inserted into the interior of the skull through a drillhole. If a second hole is drilled to insert a drain, the risk of trauma is obviously doubled. Thus, the need exists for a single device capable of simultaneously monitoring intracranial pressure and draining cerebrospinal fluid.
Simultaneous optical pressure monitoring and drainage assemblies are known. For instance, the Camino intensity modulation sensor can be used with an auxiliary drain inserted into the skull through a bolt. However, the assembly is subject to breakage, requires a large drillhole to accommodate the bolt, and is assembled out of standard connectors that are subject to leakage and, therefore, provide sources of infection. However, an optical pressure monitoring and drainage assembly is highly desirable because the accuracy of fluid column intracranial sensors is destroyed by simultaneous drainage of cerebrospinal fluids. What is needed is an integral fiber optic pressure transducer and simultaneous drainage system that interfaces with conventional monitors via a low cost processor.
All pressure monitoring systems require calibration. In pressure monitoring systems utilizing two fluid reservoirs separated by a diaphragm, such as the fluid column pressure sensor, one reservoir is usually in pressure communication with the local atmosphere, while the other, the applied pressure, is connected to the pressure source to be measured. If the atmospheric pressure is also placed momentarily on the applied pressure input, then the diaphragm moves to the zero point location, and the zero pressure offset error can be measured. However, an implanted catheter, such as an intracranial catheter, cannot be removed for calibration and replaced without introducing the risk of infection. Thus, calibration must be accomplished in situ within the brain.
One means of calibrating a pressure sensor is to provide a substitute pressure transducer system for generating a known test pressure that is displayed on the monitoring device as a calibrated output indicating the level of the known pressure. Another method is that employed by the Model CT/6FB catheter tip pressure transducer manufactured by Medical Measurements Incorporated. In this system the sensor, which includes a mechanical stop to indicate the zero pressure location of the transducer, is depressurized in vivo and calibrated using a micromanometer to provide known test pressures. Thus the zero pressure error and calibration error can be determined. For pressure sensors used in monitoring intracranial pressure, it is particularly desirable to calibrate the sensor in situ so that the sensor can be left in place for up to five days, thereby minimizing the risks of infection and the like. Therefore, the need exists for new and improved pressure monitoring systems such as a dual lumen catheter for monitoring fluid pressure via a first lumen while simultaneously withdrawing an amount of the fluid being measured or infusing a second fluid through a second lumen. And for monitoring intracranial pressure, what is particularly needed is an integrated intracranial pressure monitor and drain assembly, preferably one employing a Fabry-Perot sensor and providing an output signal to standard hospital monitors that "looks like" the signal generated by Wheatstone bridge strain gauge sensors.